Photon detectors for positron emission tomography (PET) imaging systems typically include a photo sensor element and a scintillator element. Visible (or near visible) photon(s) are emitted from the scintillator when a high energy photon created by the annihilation of a positron from a radioactive tracer strikes the scintillator. These scintillation photons can be detected by the photo sensor(s). Signals from the photosensors representing the annihilation photon event are combined and used to create images of a region-of-interest undergoing imaging.
Conventional PET scanners can utilize a silicon photo multiplier (SiPM) as the light sensor. A SiPM can be an array of passively quenched Geiger-mode avalanche photodiodes (APD) that react to impinging photons. The SiPM can provide information about certain parameters, such as the time of the impingement event, the energy associated with the event, and the position of the event within the detector. These parameters can be determined through processing algorithms applied to the analog signals generated by the SiPM. Some conventional SiPMs can produce very fast signals, which provides a high degree of timing accuracy.
SiPMs provide certain advantages over conventional photomultiplier tubes (PMTs), and are therefore being used in many applications, including Positron Emission Tomography for medical imaging. These advantages include better photon detection efficiency (i.e., a high probability of detecting an impinging photon), better timing performance (i.e., determine the time of the arrival of the photons with higher precision), compactness, ruggedness, low operational voltage, insensitivity to magnetic fields and low cost.
A SiPM consists of a large array of microcells. A microcell contains a Single Photon Avalanche Diode (SPAD) which is a photodiode biased above its breakdown voltage. When a photon interacts in the SPAD, the diode breaks down and produces a large discharge current. In an analog SIPM, a quenching resistor is placed in series with the SPAD. As the discharge current increases, the voltage drop across the quenching resistor increases and the voltage across the SPAD decreases. When the voltage across the SPAD drops below the breakdown voltage, the discharge stops and the voltage across the SPAD will increase until it reaches the bias voltage. At that point, the SPAD is ready to detect the next photon that interacts in it. The currents thru the quenching resistor during the discharge and recharging of the SPADs in the SiPM are combined and measured to determine the time a light pulse struck the SiPM and the number of photons in the light pulse.
In a digital SiPM, circuitry is added to each microcell. The microcell circuitry processes the signal produced from a SPAD and the outputs signals from the circuits are combined to determine the time a light pulse struck the SiPM and the number of photons in the light pulse. The quenching resistor may be present or it can be replaced with a digitally controlled reset circuit.
For both the Analog and Digital SiPM, the area covered by a microcell includes the area of the SPAD, the area covered by the quenching resistor and/or microcell electronics, traces (for signals and voltages), and the spacing between microcells required for electronic isolation. Photons that hit the area of a microcell that is outside the photo sensitive area of the SPAD are generally not detected and the ratio of the photo sensitive area of the SPAD to the total area of the microcell (the Fill Factor) is a critical parameter in determine the probability that a photon which hits the SiPM will be detected (Photon Detection Efficiency: PDE).
The area covered by the electronics and/or the quenching resistor is essentially independent of the SPAD size, and the areas required for electronics isolations and traces nominally increase proportionally to the square root of the microcell area. Therefore, a common method of increasing the PDE is to increase the area of the SPAD in the SiPM.
However, increasing the size of the SPAD also adversely affects other SiPM parameters. In particular, the probabilities of producing an after pulsing events and optical cross talk events increase with the area of the SPAD. An after pulse event occurs when an electron or hole generated during the discharge of a microcell becomes trapped in an energy level of the silicon, the electron or hole is thermally released from the trap in a time that can vary from a few nanoseconds to several microseconds, and the released electron or hole initiates another discharge of the microcell. An optical cross talk event occurs when a photon produced by the discharge of a microcell travels to a neighboring microcell and initiates a discharge in that microcell. The probability of creating an after pulse event or an optical crosstalk event increase essentially linear with the area of the SPAD. The PDE of an SiPM can be enhanced by placing an optical structure on top of the microcell which directs light away from the dead space of the microcell and onto the active area of the SPAD